1. Field of the Invention
The present invention relates to a deoxidizer package and a food pack in which a food is packed in a container together with a deoxidizer package. The invention also relates to a method of freeze-storing a food, as well as a method of thawing a freeze-stored food. The invention further relates to a method of storing a food and a method of cooking a stored food.
More particularly, the invention is concerned with a deoxidizer package for use together with a food which is to be thawed or cooked by irradiation with a microwave. The invention is also concerned with a food pack which has a deoxidizer package packed together with a food and which is to be treated by irradiation with a microwave. The invention is further concerned with a thawing or cooking method in which a food packed together with a deoxidizer agent and freeze-stored or maintained at a temperature below room temperature is thawed or cooked by irradiation with a microwave. The invention is still further concerned with a method of storing a food in which a food is packed in a container together with a deoxidizer package and is stored after irradiation with a microwave.
2. Description of the Related Art
Food storage technics making use of deoxidizers have recently been established and have found spreading widespread use in storing a variety of types of foods. The food storage technique making use of a deoxidizer involves packaging a food in a container together with a deoxidizer so as to maintain an anaerobic atmosphere in the container so as to obtain various effects such as prevention of oxidation of fat content of the food, prevention of fading or discoloration of the food, preservation of flavor, protection against noxious insects, prevention of breeding of aerobic bacteria, and so forth, thereby maintaining the quality of the food. The deoxidizer is a composition capable of absorbing oxygen. Usually, the deoxidizer is used as a deoxidizer package in which the deoxidizing compostion is packaged in a small sack made of a gas-permeable material. Among various deoxidizing components which have been proposed and used hitherto, compositions containing powdered iron have been most popular because of advantages such as safety, oxygen absorptivity and cost. On the other hand, the sacks of deoxidizer packages are usually made of a laminate sheet composed of a sheet of paper and a porous polyethylene film bonded together or a laminate sheet composed of a perforated plastic film and a porous polyethylene film bonded together.
Food storage technics relying upon deoxidizers, however, cannot satisfactorily prevent degradation of food which proceeds even in an anaerobic atmosphere, such as deterioration due to actions of bacteria and enzyme and aging of a starch component and so on. Deoxidizers are also ineffective in resisting some types of noxious insects which have small oxygen demands. Namely, 100-percent extermination of such insects by a deoxidizer often requires 9 to 12 days which are much longer than that required when a smoke type insecticide is used.
As a result of the current spreading of household microwave ovens, it has become popular to heat-cook or thaw a food pack by an irradiation with a microwave in microwave ovens. Nowadays, a variety of pre-cooked packed foods are manufactured by food manufacturers and available commercially, so that the demand for heating with microwave irradiation is further increased.
It is also a current tendency that food manufacturers chill or freeze their products before supplying them to markets, for the purpose of stock control and production control, in order to cope with a fluctuation in the demand. It is also a current measure that the food manufacturers freeze-store food packs at temperatures below 0.degree. C. and thaw them by microwave irradiation before shipping. Storage at low temperatures is very effective in preventing degradation of foods due to action of enzymes, as well as in the prevention of aging of starch. Preservation of foods by sterilization, enzyme deactivation and insect killing by microwave irradiation also is a current technic. In this technic, bacteria and insects as dielectric matters are heated and killed by high-frequency induction heating caused as a result of the microwave irradiation. The treatment by irradiation with microwave can be conducted after packing of the foods to be preserved. This method, therefore, has a spreading use because of its advantages such as capability for prevention of secondary contamination, high treating speed and good treating conditions.
Thus, developments have been achieved both in the storage of foods together with deoxidizers and in the utilization of microwave irradiation for the purpose of heating, thawing, cooking, sterilization, enzyme deactivation and insect killing of foods stored at room temperature or in a chilled or frozen state. These developments have given rise to the demands for technics for packing foods together with a deoxidizer package in a container and storing the pack at a room temperature or in a chilled or frozen condition, as well as for technics for microwave irradiation of the food pack containing a deoxidizer package and stored at room temperature or in a chilled or frozen state for the purpose of heating, thawing, cooking, sterilization, enzyme deactivation or insect killing. Preferably, food packs of pre-cooked foods, which are to be heated, thawed or cooked, are charged with deoxidizer packages so that the above-mentioned demand is a matter of a great significance. It is also very important to develop a food storage technic which is capable of obviating the shortcomings of the food storage technic relying upon deoxidizer, such as breeding of facultive and strict anaerobic bacteria, degradation due to enzyme action and long time required for killing insects, as well as shortcomings of microwave irradiation type food storage technic such as ineffectiveness to fading and discoloring, as well as oxidation of oily components.
Irradiation with a microwave, however, causes a problem when applied to a food pack containing a deoxidizer package. Namely, a large proportion of the microwave penetrates the packaging sack material so as to be absorbed by the contents of the deoxidizer package, such as iron powder. In consequence, eddy currents are generated in the contents of the deoxidizer package so that heat is rapidly produced to cause various troubles such as burning of the packaging sack material or the rupture of the packaging sack due to a rapid rise of internal pressure as a result of rapid expansion of vapor produced caused by evaporation of water content of the deoxidizer. In the latter case, the deoxidizer may be scattered in the food pack so as to contaminate the food in the pack.
In order to overcome this problem, a method has been proposed in the specification of Japanese Patent Application No. 61-221131 in which a conductive material such as a metal foil is used as the packaging sack material so as to prevent the microwaves from reaching the contents of the deoxidizer package. This proposal, however, is still unsatisfactory in that electric sparking is generated as a result of eddy currents induced in the packaging sack material so that the sack is broken to allow the contents to be scattered, thus causing the same problem as that described above.
Thus, known deoxidizer packages are not resistant to microwave irradiation so that they could not be used in food packs which are intended for heating, thawing, cooking, sterilization, enzyme deactivation and insect killing by irradiation with a microwave.